Diabetes is a disease characterized by an impaired glucose metabolism manifesting itself among other things by an elevated blood glucose level in untreated diabetic patients. The underlying defects lead to a classification of diabetes into two major groups: type 1 diabetes and type 2 diabetes. In type 1 diabetes or insulin demanding diabetes mellitus (IDDM), the patients lack β-cells producing insulin in their pancreatic gland. Type 2 diabetes or non-insulin demanding diabetes mellitus (NIDDM), occurs in patients with an impaired β-cell function besides a range of other abnormalities. Type 2 diabetes may eventually develop into type 1 diabetes. While insulin treatment of patients suffering from type 1 diabetes is indispensable it may also be advantageous in the treatment of type 2 diabetes in some cases.
Since the discovery of insulin in the 1920's, continuous strides have been made to improve the treatment of diabetes mellitus. To help avoid extreme glycaemia levels, diabetic patients often practice multiple injection therapy, whereby insulin is administered with each meal. Many diabetic patients are treated with multiple daily insulin injections in a regimen comprising one or two daily injections of a protracted insulin composition to cover the basal requirement, supplemented by bolus injections of a rapid acting insulin to cover the meal-related requirements.
Insulin compositions having a protracted profile of action are well known in the art. Thus, one main type of such insulin compositions comprises injectable aqueous suspensions of insulin crystals or amorphous insulin. Typically, the insulin in these compositions is provided in the form of protamine insulin, zinc insulin or protamine zinc insulin.
When human or animal insulin is brought to form higher associated forms, e.g. in the presence of Zn2+-ions, precipitation in the form of crystals or amorphous product is the result; see, for example, pages 20-27 in Jens Brange (editor), Galenics of Insulin, Springer Verlag (1987). Thus, at pH 7, addition of 6 Zn2+ ions per insulin hexamer to a solution of porcine insulin will lead to an almost complete precipitation of the insulin.
Another type of protracted insulin compositions can be obtained with insulin analogues that are water soluble at pH values below physiological pH but not at physiological pH. When a solution of such an insulin analogue is injected, the insulin analogue will precipitate to form a subcutaneous depot of solid material because of the rise in the pH value to physiological pH. This principle may be combined with the present invention by incorporation of the glucose-sensor in the insulin analogue. In addition to the glucose sensor these analogues have an amino acid residue in position A21 that is stable at pH values as low as practically useful in solutions to be injected. Examples of suitable amino acid residues at position A21 are glycine, serine and alanine. Also, the insulins have mutations to increase the net charge of the molecule by about 2 units, e.g. Thr in position B27 can be substituted with Arg and Thr-OH in position B30 can be substituted with Thr-NH2 or basic residues can be added, e.g. B31-B32 Arg-Arg.
Soluble insulin derivatives having a lipophilic substituent linked to the ε-amino group of a lysine residue in any of the positions B26 to B30 have been described in the literature. Such derivatives have a protracted profile of action after subcutaneous injection as compared to soluble human insulin, and this protracted action has been explained by a reversible binding to albumin in subcutis, blood and peripheral tissue.
An additional mechanism of prolonging the action of some of the soluble insulin derivatives featuring a lipophilic substituent has been disclosed, i.e. derivatives capable of forming high-molecular-weight aggregates, having a higher molecular weight than aldolase (Mw=158 kDa) when analysed in a specified gel filtration system (WO 99/21888, Novo Nordisk).
In healthy persons, the blood glucose concentration is about 5 mM, rising to about 7 mM after the meals. Today, even when applying the most advanced insulin treatment, using rapid acting insulins for meal-related injections and soluble depot insulin for basal insulin based on frequent monitoring of blood glucose, diabetic patients often experience glucose concentrations out of control. If too much insulin is administered, so that glucose concentrations get below about 3 mM, hypoglycaemic events may occur. When too little insulin is administered and glucose concentrations rises to about 20 mM, acetone appears in the blood and gives rise to diabetic ketoacidosis and, eventually, diabetic coma. In order to avoid these complications and also in order to minimize the occurrence of diabetic late complications it is desirable to control the blood glucose concentration of diabetic patients to be as close to 5 mM as possible. The DCCT (Diabetes Complication Clinical Trial) study from 1993 in USA examined the development of diabetic complications in type 1 diabetic patients during 9 years (N Engl J Med 1993, 329, 977-986). The UKPDS (United Kingdom Prospective Diabetes Study) studied the development of complications in type 2 diabetic patients during 15 years (Lancet 1998, 352, 854-865). Even though the pattern of complications differs between these two types of diabetic patients both investigations conclude that a tight control of blood glucose results in a marked reduction of complications. Thus, there is an unmet need for means to obtain glucose control in diabetic patients closer to the normal value of 5 mM.
In theory, one way to obtain tight glucose control would be to couple a glucose sensor, positioned in the tissue of the patient, to a computer that controls an insulin pump. The pump is via a catheter connected to a needle inserted under the skin. However, it appears as if such a feed back control system has not yet been implemented, possibly because of lack of stable and reliable of glucose sensors. Glucose sensors inserted in the tissue appear to get overgrown with fibrin within a very short time, and it appears that suitable non-invasive sensors, e.g. based on infrared optics, remain to be invented or developed.
Attempts to develop systems for glucose dependent release of insulin from a depot has previously been described. A carbohydrate binding lectin, such as concana-valin A, immobilized to a solid matrix, such as hollow fibres, binds an insulin derivative substituted with a carbohydrate moiety, such as maltotriose, maltose or dextran. The matrix allows diffusion of dissolved glucose and insulin derivative. As the systemic glucose concentration rises, glucose displaces increasing amounts of the insulin derivative from the matrix, thus making more insulin available to the circulation, and thereby to the insulin receptors, when it is needed. It appears as if none of these lectin based systems have been implemented clinically, probably due to the inconvenience of implanting the insulin containing matrix in the body, and to the danger of carrying a large insulin depot within the body.
Another suggested glucose-controlled insulin release system is based on the glucose oxidase catalysed conversion of glucose to gluconic acid. The glucose oxidase is immobilized to a matrix, e.g. of ethylene/vinyl acetate copolymer, and the insulin or insulin derivative is trapped in the matrix in the solid state. As the pH is lowered locally due to the production of gluconic acid the solubility of insulin increases. Thus, the rate of release of soluble insulin from the solid state reflects the glucose concentration. Likewise, it appears as if none of these glucose oxidase based systems have been implemented clinically, possibly for the same reasons.
Furthermore, attempts to provide glucose controlled insulin release from a depot in which the glucose sensing molecular structure is part of a matrix, i.e. a soluble or solid polymer have been made.